Ubiquitin is a small protein that has important regulatory roles in a wide variety of cellular pathways. The best known of these is ubiquitin's role in protein degradation, where covalent attachment of ubiquitin to a target protein enables that targeted protein to be recognized and destroyed by the 26S proteasome (see Wilkinson, Semin. Cell Devel. Biol. 11(3): 141-148 (2000)). The covalent attachment of ubiquitin, a 76 amino acid protein, to a target protein is a three-step enzymatic process (Pickart, Annu. Rev. Biochem. 70: 503-533 (2001)). First, ubiquitin-activating enzyme E1 forms an ubiquitin-E1 thioester in an ATP-dependent reaction. The ubiquitin is transferred from the ubiquitin-E1 thioester to a member of the ubiquitin-conjugating enzyme (E2) family in the second step. In the third step, with the assistance of a ubiquitin-protein ligase (E3), an isopeptide bond is formed between the carboxyl terminus of ubiquitin and the ε-amino group of a lysine residue on the target protein. Enzymes termed deubiquitinases remove ubiquitin moieties from target proteins (Guterman and Glickman, Curr. Prot. Pep. Sci. 5: 201-210 (2004)).
Ubiquitin contains seven lysine residues (Lys6, Lys11, Lys27, Lys33, Lys29, Lys48, and Lys63), and thus ubiquitin itself may serve as a target protein for ubiquitination (Peng et al., Nat. Biotechnol. 21:921-926 (2003); Pickart and Fushman, Curr Opin. Chem. Biol. 8:610-616 (2004)). The molecule produced upon ubiquitination of a ubiquitin protein is termed a polyubiquitin molecule, and may comprise two or more ubiquitin moieties. Ubiquitination of ubiquitin may theoretically occur at any of the seven lysine residues (Peng et al., Nat. Biotechnol. 21: 921-926 (2003)), so that different species of polyubiquitins exist having isopeptide bonds to different lysine residues within ubiquitin. Polyubiquitin chains with internal isopeptide linkages at all seven lysine resides have been reported. Iwai and Tokunaga, EMBO Reports 10:706-713 (2009).
Recently it was discovered that linear polyubiquitin chains also form in which the C-terminal glycine of ubiquitin is conjugated to the α-amino group of the N-terminal methionine of another ubiquitin molecule. Iwai and Tokunaga, EMBO Reports 10:706-713 (2009). Linear polyubiquitin is formed via the linear ubiquitin chain assembly complex (LUBAC) which is composed of two ring finger proteins, HOIL-1L and HOIP. Tokunaga et al., Nat. Cell Biol. 11:123-132 (2009). It is believed that genetically encoded, unanchored linear polyubiquitin does not exist in cells as its C-terminus is vulnerable to cleavage by isopeptidase T. Iwai and Tokunaga, EMBO Reports 10:706-713 (2009). This observation suggests that linear polyubiquitin is assembled onto a substrate protein post-translationally and that conjugated linear polyubiquitin molecules are potential modulators of protein activity and function. Id. For example, linear polyubiquitination of the NF-kB essential modulator (NEMO) has been shown to play a role in NF-κB activation. Id.
Antibodies which distinguish linear polyubiquitin over polyubiquitin of different lysine linkages would be useful to further examine the role of linear polyubiquitin chains in protein degradation and regulation and to target and modulate linear polyubiquitin in linear polyubiquitin-mediated pathways.